Why do Christians celebrate Christmas on December 25?

People often ask me why Christians celebrate the birth of Jesus on December 25. Actually, some Christians celebrate it on January 6, which I personally like because it’s my birthday. But I digress.
Nobody knows for certain how December 25 (and January 6) came to be the dates to celebrate Christmas. The most commonly accepted theory is that the dates were chosen, perhaps by Emperor Constantine, to divert attention from pagan holidays on the same days. Emperor Aurelian had established a winter solstice festival on December 25, A.D. 274, apparently dedicating a temple to the sun god on that date. Egyptians celebrated the birthday of the god Aion on January 6.
Some writers believe the day was chosen because it was nine months after the month and day of Jesus’ death, and Christians wanted to celebrate his conception on the same date as his death. Why the difference in December 25 and January 6? In the East, the Jewish date of 14 Nisan, the date of Christ’s crucifixion, was thought to be on April 6, but in the West, it was thought to be on March 25. If this theory is correct, then the date may have been chosen without any connection to pagan celebrations.
One thing that is certain is that the earliest Christians did not celebrate Christmas as we know it today. The Gospels say nothing about the date of Christ’s birth. It seems unlikely the shepherds would be outdoors watching the flocks at night in the winter (Luke 2:8). In the year A.D. 243, the church father Cyprian theorized that Christ’s birth should be celebrated on March 28, the spring equinox, for “on that day on which the sun was made on the same day was Christ born.” The oldest reference to Christmas occurs in a Roman church calendar in A.D. 354. By A.D. 376, the Roman bishop was requiring churches to celebrate Jesus’ birth on December 25, but eventually Eastern Christians celebrated Christ’s birth on January 6, as many still do to this day.

from the time of the winter solstice, the amount of daylight steadily increases.
And this honors that Christ the Light of the World has come among us.

Conversely, the time of the Summer Soltice was chosen by the Church for the celebration of the birth of St. John the Baptist, who said ‘I must decrease’,
and it is at that time of year that the light of day slowiy begins to decrease.